1. Field of the Invention
The present invention relates in general to digital/analog converter circuits for converting digital signals into analog signals, and more particularly to a digital/analog converter circuit using a current switching manner with the same signal transfer time with respect to devices in the circuit.
2. Description of the Prior Art
Generally, digital information can readily be processed, and must be converted into analog information so that it can easily be recognized. To this end, a digital/analog converter circuit is used to convert the digital information into the analog information. The digital/analog converter circuit may generally be applied to the process of audio and video signals and a control system based on an analog value. The digital/analog converter circuit is required to have a high speed processing ability and a high precision, particularly in the video signal process application.
In a conventional digital/analog converter circuit, an input digital signal is converted into a discrete control signal by a decoder. The discrete control signal remains at its present state until the subsequent digital signal is inputted, resulting in output of successive analog signal values.
Referring to FIG. 1, there is shown a circuit diagram of a conventional digital/analog converter circuit of the segment drive type. As shown in this drawing, the conventional digital/analog converter circuit comprises a transistor M1 having a gate terminal for inputting a constant voltage Vref1 and a source terminal for inputting a power source voltage Vaa. As a result, the transistor M1 is operated as a saturation current source. A constant voltage Vref2 is applied to a gate terminal of a transistor M3, whereas a digital input signal Di1 is applied to a gate terminal of a transistor M2. As a result, the transistors M2 and M3 are switched to the opposite states according to a state of the digital input signal Di1.
However, in the above-mentioned conventional digital/analog converter circuit of FIG. 1, a voltage at an output node n1 of the transistor M1 is shaken within a certain range because the signal flow of the transistor M2 is controlled by only the digital input. This is a barrier to the improvement in a switching speed.
Referring to FIG. 2, there is shown a circuit diagram of another conventional digital/analog converter circuit of the segment drive type, which has been made in view of the above problem with the conventional digital/analog converter circuit of FIG. 1. As shown in this drawing, the conventional digital/analog converter circuit comprises an inverter I1 for inverting the digital input signal Di1 and applying the inverted digital input signal to the gate terminal of the transistor M3. The use of the inverter I1 causes the digital signals of the opposite logical levels to be applied to the gate terminals of the transistors M2 and M3. This has the effect of preventing the voltage variation at the node n1.
However, in the above-mentioned conventional digital/analog converter circuit of FIG. 2, the digital signals of the opposite logical levels are not arrived simultaneously at the gate terminals of the transistors M2 and M3. As a result, the transistors M2 and M3 may simultaneously be turned on or off, resulting in generation of a glitch signal. The generation of the glitch signal results in a delay in a signal conversion speed.